Sheet buffering method

Abstract: A method of sheet buffering with a sheet buffering apparatus that includes at least one pair of stationary spaced apart curvlinear guide members (26,28) defining a sheet travel path (P), and a first paper nip (20) downstream of the at least one pair of stationary spaced apart curvlinear guide members (26,28) in the sheet travel path (P), the method comprising: feeding the leading edge of a first sheet (S 1 ) into said first paper feeding nip (20); moving the trailing edge of said first sheet (S 1 ) laterally off of said sheet travel path; and urging a leading edge of a second sheet (S 2 ) into said pair of spaced apart guide members (26,28) and along said sheet travel path (P) lapping the trailing edge of said first sheet (S 1 ).

Description: The subject invention is directed to the art of paper sheet handling and, more particularly, to a sheet buffering method. The inventive method is especially suited for use in the paper handling and transport system of an electrophotographic printing machine and will be described with reference thereto; however, as will become apparent, the invention could be used in many types of paper sheet handling systems in a variety of different machines. In electrophotographic printing machines, it is sometimes necessary or desirable to tem-temporarily hold or delay the transport of individual paper sheets at various points in the paper path to provide additional time for downstream processing to be performed. Such temporary holding or delaying of sheets is generally referred to as "buffering" and has been accomplished in many different ways. One prior art method of buffering is to temporarily slow or stop a roll nip or other paper transport for a period of time equal to the inter-copy-gap between successive sheets. Of course, this yields only a very short buffering time. If longer times are required, other systems must be used. For example, multiple path systems and systems which run sheets against stalled roll pairs or stop gates have sometimes been used. US-A-4,801,134 discloses a paper feed apparatus capable of feeding common use papers and specifically processed papers, comprising a feed out roller 16, a regular rotational roller 20 for feeding out sheets S one by one towards a conveying roller pair 30,40 at a downstream position along a paper path defined by paper guides. US-A-2,979,330 dicloses a sheet separating device for positively separating sheets of paper, comprising pairs of axles 18,19 with corresponding sets of nearly abutting rollers 17, or (Fig. 2) a pair of axles 41,42 provided with staggered sets of rollers 40. EP-A-0303276 discloses a sheet conveying method comprising: providing a first paper feeding nip 112, a second paper feeding nip 114 downstream of said first paper feeding nip 112, and stationary spaced apart guide members 50,51,52 defining a sheet travel path between said first paper feeding nip 112 and said a second paper feeding nip 114; feeding a trailing edge of a first sheet through said first paper feeding nip 112 and a leading edge of said first sheet into said second paper feeding nip 114; moving said trailing edge of said first sheet laterally out of said sheet travel path and holding said first sheet stationary by said second nip 114; and feeding a leading edge of a second sheet through said first paper feeding nip 112 and along said sheet travel path to at least partially lap said first and second sheets in said sheet travel path. The method of the present invention may be performed with a paper transport system which provides a simple and effective buffering system which uses only one paper path and maintains positive drive on the sheets at all times. in particular, it may be performed with a paper transport system wherein paper sheets are moved seriatim along a predetermined course, comprising first and second fiction surfaces mounted for movement in orbital paths on opposite sides of the predetermined course in opposed relationship to define a nip through which the paper sheets pass, selectively operable drive means for controlling the movement of the first friction surface, and the first friction surface having a coefficient of friction with the paper sheets which is greater than the coefficient of friction of the second surface with the paper sheets which is in turn greater than the coefficient of friction between the paper sheets. Preferably, each of the first and second friction surfaces is continuous throughout its respective orbital path. Preferably, each of the first and second friction surfaces has substantially constant friction characteristics throughout its full extent. Preferably, the drive means is capable of selectively stopping and reversing the direction of movement of the first and second friction surfaces about their respective paths of orbital movement. Because of the noted relationship between the coefficients of friction of the first and second friction surfaces, the apparatus allows two sheets to be stopped and held in the same nip and then fed out either independently or simultaneously. Note that when a first sheet is in the nip, the first friction surface can be stopped while the second friction surface continues to be driven. The first sheet will, however, be stopped and frictionally held by the first friction surface. A second sheet can then be fed between the stopped first sheet and the second friction surface. By then stopping movement of the second friction surface, both the first and second sheets are held in the nip. Alternatively, by selectively driving both or a selected one of the friction surfaces, both or a selected one of the sheets can be driven from between the nip. The present invention provides a method of sheet buffering according to claim 1 of the appended claims. Preferably, the step (B) includes the step of bending the first sheet with said curvilinear sheet travel path; and the step (C) comprises releasing said first sheet from said bending after said trailing edge is released by said first nip and releasing said trailing edge from said sheet travel path. The step (A) of defining said curvilinear sheet travel path may include the steps of (A1) defining a fi rst sheet travel path portion tangential to said first paper feeding nip in a first direction by providing said first paper feeding nip in a first orientation; and (A2) defining a second sheet travel path portion tangential to said second paper feeding nip in a second direction by providing said second paper feeding nip in a second orientation different from said first orientation. The step (B) may include the step of bending the first sheet against said at least one sheet guide member in substantial conformity with said curvilinear sheet travel path; and the step (C) may include the step of relaxing the trailing edge from said bending into a baffle on said at least one sheet guide member. The step (C) may include the step of urging the trailing edge against a first of said spaced apart guide members using a spring member on a second of said spaced apart guide members. For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following drawings, in which: FIGURE 1 is a schematic elevational view of a portion of a paper transport system FIGURES 2 to 5 are partial schematic elevational views similar to FIGURE 1 but showing a possible sequence of steps in using the apparatus of FIGURE 1; FIGURE 6 is a schematic showing of a modified form of sheet buffering system; FIGURE 7 is side elevational view of the FIGURE 6 example; FIGURES 8 to 12 are identical side views of key portions of a sheet buffering system according to a further example showing sequential steps in the operation thereof; and FIGURES 13 to 15 show three alternative upstream feeder modifications of the buffer system of Figs. 8 to 12. In the drawings, like reference numerals have been used throughout to designate identical elements. FIGURE 1 schematically depicts a portion of a sheet transport system having a sheet buffering arrangement. The system shown in FIGURE 1 is specifically intended for use in an electrophotographic printing machine; however, the apparatus and system could clearly be used in a variety of other types of equipment incorporating sheet handling and transportation systems. Broadly, as illustrated in FIGURE 1, the apparatus generally comprises guide means which define a predetermined course of paper movement or path indicated generally by the dash dot line P . In the preferred configuration the guide means comprise a spaced pairs of respective upper and lower guide panels 12 and 14 , respectively, which direct sheets to a first pair of horizontally positioned driven rolls 16 and 18 , respectively. The rolls 16 and 18 are positioned in opposed relationship and driven in the direction of the arrows to define a first drive nip 20 . The buffering station 24 is located immediately downstream of the drive rolls 16, 18 and includes upper and lower sheet guides 26 and 28 which are positioned in spaced opposed relationship and arranged to direct sheets coming from the drive nip 20 downwardly into the nip 30 of a second pair of spaced rolls 32 and 34 , respectively. Sheets passing through the nip 30 are received and directed along the predetermined path of paper movement to subsequent use or processing equipment (not shown) by suitable guide means in the form of guide plates or panels 36 and 38 . For reasons which will subsequently become apparent, the rolls 32 and 34 are each provided with separate drive means capable of independent operation. Also, roll 32 is equipped with a braking means. In the FIGURE 1 showing, the drive means are depicted schematically and could comprise any standard type of drive motor. As illustrated, roll 32 is provided with a first independent drive means 40 , the roll 32 having an associated brake. Roll 34 is driven in a similar manner from an independent drive means 42 . The drive means 40 and 42 are controlled in a manner subsequently to be described from a. main controller unit 44 . Suitable sheet sensors 46 and 48 are positioned immediately downstream of the rolls 16 and 18 to detect the presence of sheets entering the buffering station 24 . The system and apparatus shown in FIGURE 1 allows two sheets to be stopped in the buffer station 24 and held in nip 30 and then fed out either independently or at the same time While this function is being carried out, the system maintains a positive drive on the sheets at all times. To accomplish this function and result, the system is arranged so that the rolls 32 and 34 have a particular relationship in their coefficient of friction relative to the paper being handled. Specifically, the roll 32 is a high friction roll and has a coefficient of friction relative to the paper being transferred which is higher than the coefficient of friction of roll 34 relative to the paper being transferred, and both rolls have their coefficient of friction selected so that they are higher than the coefficient of friction of the paper to paper. Although many different materials could be used to form the rolls 32 and 34 to have the required relative coefficient of friction, in the subject example, the low friction on roll 34 is made of microcellular urethane with a coefficient of friction to paper of approximately 1.2, and the high friction roll 32 has a surface coating of an RTV silicone with a coefficient of friction to paper of approximately 2.5. The soft, compressible nature of the microcellular urethane yields a wide nip zone. This distributes the nip force over a large area resulting in low pressures that minimize the forces that can cause image smear. With the rolls 32 and 34 having the relative coefficient of friction in the ranges as described, the system can be operated generally in the manner best understood by reference to FIGURES 2 to 5. For example, as shown therein, a first paper sheet S 1 is driven into the buffering nip 30 by being directed thereto from rolls 16 and 18 . At the time the sheet S 1 is directed to the buffering nip 30 , both rolls 32 and 34 are being driven from their respective drive means 40.42. As the trailing edge of the sheet S 1 passes the sensors 46, 48, the controller 44 acts to stop the driving movement of roll 32 (the roller having the highest coefficient friction relative to the paper). Also, the brake (not shown) is simultaneously actuated to hold roll 32 in its stopped position. Roll 34 continues to be driven but the sheet S 1 remains stationary in the position shown in FIGURE 3 because of the significantly higher coefficient of friction between the sheet and the stopped roll 32 . Of course, the driven roll 34 merely continues rotating and slips on the surface of sheet S 1 . It should be noted as shown in FIGURE 3 that when the sheet S 1 has moved to the stopped position, the trailing edge is preferably in an upper or raised position as permitted by the shape of upper guide plate 26 and the action of a Mylar leaf spring (not shown). This places the first sheet S 1 in a position such that the second sheet to enter the buffering station 24 from rolls 16, 18 will enter a position beneath sheet S 1 . This is shown in FIGURE 4 wherein the second sheet S 2 is engaged between the driven roller 34 and the first sheet S 1 . Because of the relatiohships between the various coefficient of friction, the sheet S 2 is driven along the path and slides along on the first sheet S 1 which maintains its stationary position against the stopped roll 32 . With the second sheet S 2 fed into the nip 30 , both rolls can be actuated to drive both sheets S 1 and S 2 out of the buffering station simultaneously. Alternatively, it is, of course, possible to maintain roll 32 in itsstopped position and merely drive the second sheet S 2 through nip 30 while maintaining sheet S 1 in its stopped position. Of course, it is also possible to stop roll 32 for a predetermined period of time to hold both sheets S 1 and S 2 in position in nip 30 . Thereafter, the rolls 32 , 34 can be actuated to drive either or both of sheets S 1 and S 2 from the nip 30 . FIGURES 6 and 7 show an example wherein a single set of drive rolls in combination with superposed idler rolls can hold a first sheet while a second sheet is driven into the nip between the driven and idler rolls. More particularly, as illustrated, the FIGURE 6 embodiment includes a first set of opposed rolls 54 and 56 which define a feed nip and are driven in the direction shown to feed paper sheets S 1 and S 2 along the path 58 to a buffering station 60 . The buffering station 60 includes support and guide baffles 62 and 64 which confine the sheets moving along path 58 and direct them into the corrugation roll arrangement 66 . The corrugation roll arrangement 66 comprises a lower set of rolls 68 that are driven from a suitable drive 70 . A superjacent set of idler rolls 78 are positioned so that individual ones of the idler rolls 78 are axially intermediate the drive rolls 68 and slightly interleaved therewith to cause the sheets S 1 , S 2 therebetween to be corrugated and gripped. In operation, the first sheet S 1 to enter the buffering station 60 is stopped at the location shown by stopping the corrugation drive rolls 68 . These rolls have the high coefficient of friction and preferably have the construction as described relative to rolls 34 of the FIGURES 1 - 5 embodiment. The first sheet S 1 is thus held in the position shown, as the second sheet S 2 enters the buffering station 60 . With sheet S 1 held in position, second sheet S 2 is driven into the corrugation nip between sheet S 1 and the idler rolls 78 . When the second sheet S 2 is substantially even with sheet S 1 , the controller actuates the drive rolls. Both sheets S 1 and S 2 are then driven through the corrugation nip since sheet S 2 is loaded against sheet S 1 by the idler rolls 78 . This generates a positive drive force on the sheets, yet little drag force. Figures 8 to 12 show a buffer system 11, for a printer, which is also useful for understanding the invention. The buffer system 11 has has main sheet output path 12 defined by a downstream upper baffle 13a, an upstream upper baffle 13b, a downstream lower baffle 14a, an upstream lower (buckling) baffle 14b, a downstream feed nip 16 (or 15) at the downstream end, and an upstream feed nip 22 at the upstream end. There is less than one sheet dimension between these two feed nips 16 and 22. A buckle chamber 30 starts just downstream of nip 22, and is shown between upper baffle 13b and 13a in this example.. Alternatively, the buckle chamber 30 can be below the main path 12, as discussed herein. The buckle chamber 30 provides a substantially opening away from the main sheet path 12 for a sheet buckle to form therein. As shown in the examples of Figs. 13,14 and 15, several feeder alternatives can be used for the upstream drive nip 22 to even better ensure that the trail edge of the first sheet 18 is so buckled positively and correctly. Although conventional feed nips 22 with rollers 22a may be used, as shown, various configuration variations for the upper roll of the upstream feed nip 22 are possible to assist buckling. In Fig. 13, foam rolls 22b are inter-positioned between the upstream nip 22 normal elastomer upper drive rolls 22a. In the alternative of Fig. 14, small. paddle-blades on rolls 22c (toothed rolls) are positioned between the normal upper drive rolls 22a. In the alternative of Fig. 15, a small toothed belt 23 is used to provide continuous driving surface and ensure that the tail edge of the sheet is driven into its buffer position correctly. Further specific details and advantages of the buffer system 11 illustrated in Figs. 8 to 15 are to be found in US-A-5,289,251.

Claims: A method of sheet buffering comprising: (A) providing a first paper feeding nip (20), a second paper feeding nip (30) downstream of said first paper feeding nip (20), and at least one pair of stationary spaced apart curved guide members (26, 28) defining a curvilinear sheet travel path (P) between said first paper feeding nip (20) and said a second paper feeding nip (30); (B) feeding a trailing edge of a first sheet (S1) through said first paper feeding nip (20) and a leading edge of said first sheet (S1) into said second paper feeding nip (30); (C) moving said trailing edge of said first sheet (S1) laterally out of said sheet travel path (P) and holding said first sheet (S1) stationary by said second nip (30); and (D) feeding a leading edge of a second sheet (S2) through said first paper feeding nip (20) and along said sheet travel path (P) to at least partially lap said first and second sheets in said sheet travel path (P); wherein the step (C) of moving the trailing edge of said first sheet (S1) laterally out of said sheet travel path (P) includes the step of urging the first sheet (S1) into a step portion defined in the first guide member (26) of said at least one pair of stationary spaced apart curved guide members (26, 28). The method of sheet buffering as in claim 1, wherein the step (B) includes the step of bending the first sheet with said curvilinear sheet travel path; and the step (C) comprises releasing said first sheet from said bending after said trailing edge is released by said first nip and releasing said trailing edge from said sheet travel path. The method of sheet buffering as in claim 1 or 2. wherein the step (A) of defining said curvilinear sheet travel path includes the steps of (A1 ) defining a first sheet travel path portion tangential to said first paper feeding nip in a first direction by providing said first paper feeding nip in a first orientation; and (A2) defining a second sheet travel path portion tangential to said second paper feeding nip in a second direction by providing said second paper feeding nip in a second orientation different from said first orientation. The method of sheet buffering as in any one or the preceding claims, wherein the step (B) includes the step of bending the first sheet against said at least one sheet guide member in substantial conformity with said curvilinear sheet travel path; and the step (C) includes the step of relaxing the trailing edge from said bending into a baffle on said at least one sheet guide member. The method of sheet buffering as in any one of the preceding claims, wherein the step (C) includes the step of urging the first sheet (S1) into said step portion using a spring member (50) disposed on the second guide member (28) of said spaced apart guide members (26, 28).

Inventors: BARRIENTOS FRANCESCA A, DESANCTIS ANTHONY T, KAMPRATH DAVID R, MANDEL BARRY P, RIZZOLO CHARLES D, VANDONGEN RICHARD A, BARRIENTOS, FRANCESCA A., DESANCTIS, ANTHONY T., KAMPRATH, DAVID R., MANDEL, BARRY P., RIZZOLO, CHARLES D., VANDONGEN, RICHARD A.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1994
Main classification: B65H5
IPC codes: B65H29